The present invention relates to an electrostatic copying machine comprising an improved variable magnification optical system.
In an electrostatic copying machine of the present type, a light image of an original document is focussed onto a photoconductive drum creating an electrostatic image thereon through localized photoconduction. A toner substance is applied to the drum to develop the electrostatic image into a visible toner image, which is transferred and fixed to a copy sheet to provide a permanent reproduction of the original document.
In forming the electrostatic image, an elongated light image of a portion of the document is focussed through an aperture or slit onto the drum. The document or various members of the optical system are moved at a speed corresponding to that of the drum to scan the document.
In prior art optical systems which provide variable magnification, typically allowing a full size reproduction and one or more reduction ratios, the document must be placed at various positions on a document supporting platen depending on the magnification. This is a nuisance and the cause of many unacceptable copies produced by unskilled machine operators. For this reason, variable magnification optical systems have been developed which allow the document to be placed in the same position on the platen regardless of the desired magnification.
Such systems have heretofore not been perfected, however, and have certain drawbacks. The most serious is, perhaps, imprecise exprecise exposure control through varying the width of the slit. The correct slit width is a function not only of the intensity of the illuminating light source and drum photoconductive sensitivity but also of the magnification. Heretofore developed variable magnification optical systems for copying machines do not comprise means for effective exposure control and produce many unsatisfactory copies for this reason.
Another problem resides in the fact that the light image intensity drops off at the edges of the image. This has been compensated for in the prior art by making one aperture plate defining the slit curved in such a manner that the width of the slit or aperture is wider at the ends thereof. However, this expedient does not allow the document to be positioned on the platen such that a side edge thereof is always at the same position regardless of magnification, since the copies thus produced will be darker at one side edge than the other depending on the magnification. Still another problem which has remained heretofore unsolved is that of preventing exposure of portions of the drum in which the image would not be transferred to an undersized copy sheet.